The invention relates to loudspeakers of the kind in which an acoustic output is produced by applying bending waves to an acoustic radiator comprising a resonant member, e.g. a panel-form member, to cause it to resonate. Such loudspeakers are described in International patent application WO97/09842 of Verity Group plc later assigned to New Transducers Limited.
A vibration exciter for exciting such a resonant member needs a form of fixture onto the member to allow the best possible conversion of drive power into bending waves. An inertial reaction exciter applies a push/pull force to the member by reacting against the inertia of the driver mass and can be suitable.
FIGS. 1 and 2 show a known form of resonant panel-form loudspeaker (10) with one known kind of inertial reaction vibration exciter or transducer (see FIG. 11b of International application WO97/09842). (FIG. 1 is schematic, whilst FIG. 2 gives sectional detail of the exciter). Thus while FIG. 1 shows parts of the exciter exploded apart in the interests of clarity, it will be clear from FIG. 2 that in practice the parts of the exciter are closely arranged. The panel loudspeaker (10) comprises a vibration exciter (12) which is attached to one side of a stiff lightweight resonant panel (14). The exciter (12) includes coil winding (16) which is rigidly fixed, e.g. by means of an adhesive, on the outside of a coil former (18) to form a voice coil assembly (20) which is rigidly bonded to surface skin (22) of the panel (14), e.g. by means of an epoxy adhesive bond. Magnets (24) are enclosed by a pair of poles (26), one of which is disc-like and is disposed with its periphery close to the interior of each coil former (18), and the other of which has peripheral flange (28) arranged to surround the coil assembly (20). The magnet assembly (24,26) is secured to the surface of the panel (14) by means of a resilient suspension (30), e.g. of rubber, which is attached to the periphery of the flange (28) of the outer pole piece (26).
FIG. 3 illustrates another known resonant panel-loudspeaker (32), (see FIGS. 7a,7b,7c of International application WO97/09842) comprising an exciter (12) which is attached to one side of resonant panel 14). The exciter (12) is similar to that described with reference to FIGS. 1 and 2, in that it has a voice coil assembly (20) and magnet assembly (24,26). The voice coil assembly (20) is rigidly coupled to the panel (14) and the magnet assembly is secured to a frame (34) and resiliently rigidly secured to the panel (14) by means of a resilient suspension (30), e.g. of rubber. In practice, a resilient suspension is disposed around the periphery of the panel (14) and is coupled between the panel (14) and the frame (34), but in the present drawing this has been omitted for simplicity. By rigidly coupling a frame (34) to the magnet assembly (24,26), the advantages and disadvantage noted hereinafter may result.
A resonant panel loudspeaker driven by an electro-dynamic exciter has a substantially flat sound pressure level response with frequency. There will, however, be a frequency below which the drive force to the panel will fall. It is possible to reduce this frequency and hence extend the bandwidth of the panel loudspeaker by increasing the inertia of the exciter magnet assembly. This may be achieved simply by adding more mass to the exciter magnet assembly or alternatively by coupling the exciter magnet assembly to a more massive body, for example to a support frame although both of these approaches can be disadvantageous in some respects. Thus an increase in the inertia of the exciter renders the exciter more sensitive to damaging shock during transportation or during handling, with the possibility even of damage to the resonant panel itself, while coupling the exciter rigidly to a support causes the exciter to cease to be truly inertial and instead couples drive energy to the support.